Measurement of distance has been implemented with many different techniques, whose relative merits can be evaluated only within specific applications. For applications that preclude direct physical measurement, techniques based on time-of-flight of electromagnetic (EM) waves are often used.
One type of EM-based system uses microwaves, as implemented by surveying instruments known as Tellurometers. These instruments precisely measure ground distances by determining the velocity of a phase-modulated, continuous microwave radio signal transmitted between two instruments operating alternately as master station and remote station.
One application in which EM-based technology has been applied is that of distance measurement for topological monitoring. Many natural hazards (landslides, mud-flows, volcanic eruptions, floods, glacial surges) develop with time scales that are too short and conditions that are too dangerous for traditional field deployment. In such environments, networks of inexpensive and expendable wireless nodes have been developed to collect location information. The fusion of this new technology with evolved technologies such as high precision Global Positioning System (GPS), and other developing technologies such as Interferometric Synthetic Aperture Radar (InSAR), provide an opportunity to gather more data, more rapidly, in more hostile environments than ever before.
Although various wireless sensor node distance measurement techniques have been proposed, none approach the accuracy required for natural hazard prediction, assessment and mitigation.